The curvature of the reflective area of the reflector of known structural units of this type corresponds to a curve whose points fulfill the elliptic equation EQU 0.4604x.sup.2 +0.0483xy+y.sup.2 +11.7134x+1.0703y-263.5781=0.
In order to generate the impression that the ground-glass screen of the S8-reproduction device is uniformly illuminated, care has to be taken in the manufacture of these structural units--even more than with the known units comprising higher wattage halogen cycle incandescent lamps for the above application--that the filament of the lamp is, relative to the reflector associated with it, very precisely in the predetermined position. In other words, the range of tolerance is very small. This is a very serious difficulty, especially in quantity production, since the lamp filament must be positioned at 2/100 millimeter precisely in the burning position within the reflector. The size of the S8 film window is standardized and determined by the S8 film format. The standard distance between the window and reflector surface is 27.6 mm. The center of the light source, for example the filament, must be accurately located at 35.5 mm from the film window plane. The lamp, which has a finite size, must be fitted within the reflector. The cup-shaped or generally ellipsoid structure of the reflector, thus, is interrupted in the region of the apex thereof to permit passage of the lamp structure therethrough.